1. Field of the Invention
This invention relates to synchronization of cell sites in time division multiple access cellular networks and in particular synchronizing the transmission of frame emission times in such cellular networks from each of the nodes.
2. Description of the Related Art
In typical time division multiple access communication networks such as GSM cellular networks that are commonly found in Europe and TDMA networks that are found in the United States operating according to TIA Standard IS-54B, the geographic area of the network is divided into a plurality of cells. Located in the approximate geographic center of each cell is a base transceiver station (BTS).
According to the network protocol, each frequency channel is typically split into many separate time multiplexed channels or time slots on that frequency channel. The BTS's transmit continuously on a plurality of frequency channels in use with the frames being transmitted divided among the time slots. The mobile transceivers receive and process only one of the time slots and also transmit on one time slot in the appropriate reverse frequency channel. Because the mobile transceivers must not interfere with other mobiles using the same frequency channel within that cell site, it is necessary to synchronize all of the mobile receivers within a cell operating on the same reverse frequency channel with each other and the BTS.
Furthermore, the BTS transmits a training sequence in the middle of every time slot so that the mobile can learn the characteristics of the intervening radio path and train its equalizer. Accurate training of the equalizer is essential to properly decode the voice and other control information sent by the BTS.
However, in conventional TDMA cellular networks such as GSM, the BTS of a cell is not synchronized to the BTS of an adjacent cell. As a result, at any location in the network, a mobile will receive the training sequence transmitted from the BTS for its cell and will also receive probably at a different time the training sequence from BTS's of other cells. This is one form of what is commonly co-channel interference. It causes problems in the training of the equalizer of the mobile for training purposes.
While it might be possible to synchronize the network so that all cells are emitting their frames simultaneously, the problem with this is it requires modification to the existing infrastructure equipment in the network. Obviously, with the heavy capital investment in existing networks and the capital investment in existing BTS design, it is highly undesirable to do this.
Therefore, it is a first object of the invention to permit synchronizing the frame emission times of all of the transmitters within the network. It is a second object of the invention to permit such synchronization without modifying the existing infrastructure equipment or causing either changes in design or programming of the mobile units. It is yet a third object of the invention to permit synchronized handoffs. It is yet a fourth object of the invention to provide synchronization so that mobile units can determine their geographic location within the network.